The present invention relates to improved poly-p-phenylene-terephthalamide (hereinafter referred to as "PPTA") fibers and a process for their preparation. More particularly, the invention relates to high tenacity, high Young's modulus PPTA fibers useful for reinforcing plastics and rubbers, and a process for their preparation.
PPTA is a polymer that has been known for many years and, from the rigid molecular structure of this polymer, it has been expected that its fibers would have excellent heat resistance and mechanical properties. However, PPTA is only slightly soluble or insoluble in organic solvents. Accordingly, Cypriani proposed a basic process for wet-spinning PPTA by using concentrated sulfuric acid as a solvent (U.S. Pat. No. 3,227,793), but Cypriani's process itself was not industrialized.
It has been known for many years that when a rigid polymer is dissolved in a solvent, a liquid crystal is formed at a degree of polymerization exceeding a certain level and a concentration exceeding a certain level under a certain temperature condition, and this phenomenon has been confirmed theoretically and experimentally (P. J. FLORY; Proc. Roy. Soc., A234, 73 (1956)). It is easily predicted that if a polymer solution that is in the form of a liquid crystal and is optically anisotropic can be extruded from a nozzle and coagulated while preventing disturbance of orientation of the liquid crystal in the interior of the nozzle as much as possible, fibers having high tenacity and high Young's modulus and comprising highly oriented molecular chains will be obtained. In fact, Kwoleck proposed a process for the wet spinning of a concentrated solution of an aromatic polyamide having a rigid and linear molecular structure that is in the form of a liquid crystal (U.S. Pat. No. 3,819,587), and this type of wet spinning process again attracted attention in the art.
Blades proposed a process in which an optically anisotropic dope having an elevated concentration is extruded in air and then wet-spun to form as-spun fibers having a high tenacity owing to a specific fine-structure in the as-spun state (U.S. Pat. Nos. 3,767,756 and 3,869,429), and Blades further taught that if such as-spun fibers are heat-treated under tension, the Young's modulus can be enhanced (U.S. Pat. No. 3,869,430).
However, it has been pointed out that the above-mentioned high Young's modulus fibers to be used for reinforcing plastics or certain special rubbers have the following defects. That is, PPTA fibers having a Young's modulus elevated to a level exceeding about 600 g/d are poor in resistance to stress imposed in the lateral direction and resistance to friction on the surfaces of the fibers and, therefore, are readily fibrillated (see, for example, S. L. Fennix et al.; Textile Res. J. Dec., 934 (1974)). Accordingly, if these PPTA fibers are twisted to produce yarns practically applicable as reinforcing fibers, they are fibrillated owing to frictional contact with guides or the like and yarn dust is formed. Furthermore, when they are embedded in rubber belts or plastics as reinforcers and applied to a use where stress is repeatedly imposed, no satisfactory fatigue resistance can be attained.
Various improvements have been proposed in the preparation of high Young's modulus fibers of para-oriented aromatic polyamides inclusive of PPTA (see, for example, Japanese Patent Application Laid-Open Specifications No. 12325/77, No. 12326/77 and 98415/78), but these proposals have failed to overcome the above-mentioned problems.